For militarized compact tactical laser systems, including laser target designators and range finders, one requires a relatively high output, for instance, 170 millijoules with a beam cross-section of five millimeters. The tactical laser system is to be provided in a package that is small, has a low parts count, and in which the optics, once mounted, are not required to be adjusted.
In order to provide such efficient laser transmitters, in the past it has been the practice to pump nonlinear optical crystals within a linear cavity which establishes an optical parametric oscillator (OPO), and to use the output of the optical parametric oscillator to pump an optical parametric amplifier (OPA). Typically for these applications, one requires an eye-safe laser, with the laser pump operating in the one-micron region, whereas the nonlinear optical crystals up-convert the pump to an idler of 1.5 microns for better eye-safe operation.
In the past there has been one particularly vexing problem and that is to reduce or eliminate feedback into the pump laser. Feedback disrupts the pump laser operation and requires the use of an optical isolator, which is both very heavy and relatively large. Importantly, radiation fed back to the pump laser heats up the pump laser which is very temperature-sensitive. For a typical optical parametric oscillator having opposed cavity mirrors, the feedback is generated because a portion of the light in the cavity is reflected back through the input mirror and directly into the pump laser. The result of feedback is that Q-switching is disrupted and Q switch pulse evolution is likewise affected. This causes many problems, including high-intensity spikes, erratic, unstable output levels and damage to optical components. As mentioned above, feedback also increases the temperature of the pump laser, which severely limits the temperature range for which one can use a linear cavity.
In order to diminish or eliminate feedback from the linear cavity, the input laser beam is intentionally aligned off-axis by a few milliradians so that feedback from the linear cavity is not injected back into the pump laser.
However, locating the pump laser slightly off-axis can to some extent cure this feedback problem. While providing stable operation over temperature, providing a pump beam which is misaligned with the optical axis of the linear cavity markedly reduces the performance of the laser system by, in effect, detuning the laser. Thus, for a two- to five-milliradian misalignment required to limit feedback into the pump cavity, the result is a reduction in output of approximately 15%.
Moreover, regardless of the above intentional misalignment, when linear optical parametric oscillator cavities are utilized, there is a requirement for an optical isolator which, as mentioned above, is very heavy and does not fit into the compact systems required for military applications. It will be appreciated that the isolator is located between the pump laser and the OPO. This isolator not only extends the length of the laser transmitter because it doubles the overall length, it also adds to the thickness of the package and can add inches to the thickness. In short, the use of an isolator increases the laser platform size by a factor of two.
The isolator also adds approximately five pounds to the weight of the laser. Presently, the laser with electronics and all associated components is only approximately 16 pounds. Thus, the addition of five pounds increases the overall weight of the system by one-third.
Doubling the size of the laser system is impermissible because of the pods that are utilized on aircraft, which by and large are of a fixed size. To be able to increase the pod size to accommodate linear OPOs requires aircraft redesign and acceptance of the increased weight due to the isolator required. Use of such devices has been discouraged due to the strict weight restrictions on all weapons packages for military aircraft.
Aside from the size and weight considerations mentioned above, it is important to understand that the LIDAR or laser range finder systems are subjected to severe mechanical vibration and G-loading, which can affect the alignment of the optical elements within the LIDAR. Noting that the pump laser beam misalignment is to be carefully controlled, alignment problems become paramount in military environments. One therefore seeks not to have a system requiring this misalignment that even further deleteriously affects the laser system. Note that it is only with difficulty that one can maintain a carefully-controlled misalignment in the face of vibration and temperature swings.